Wind turbine blades, such as the long blades employed on modern utility-scale wind turbines, generally comprise a hollow blade shell made primarily of composite materials, such as glass-fibre reinforced plastic. The blade shell is typically made up of two half shells, a windward shell and a leeward shell, which are moulded separately in respective female half moulds, before being bonded together along flanges at the leading and trailing edges of the blade. This method of manufacturing a blade is illustrated schematically in FIG. 1a. 
Referring to FIG. 1a, this shows a mould 10 for a wind turbine blade divided into two half moulds, a windward mould 10a and a leeward mould 10b, which are arranged side by side in an open configuration of the mould. A windward blade shell 12a is supported on a mould surface 14a of the windward mould 10a and a leeward blade shell 12b is supported on a mould surface 14b of the leeward mould 10b. The shells 12a, 12b are each made up of a plurality of glass-fibre fabric layers, which are bonded together by cured resin.
After forming the shells 12a, 12b in the respective mould halves 10a, 10b, shear webs 16 are bonded to an inner surface 17 of the windward blade shell 12a. The shear webs 16 are longitudinally-extending structures that bridge the two half shells 12a, 12b of the blade and serve to transfer shear loads from the blade to the wind turbine hub in use. In cross-section, as shown in FIG. 1a, the shear webs 16 each comprise a web 18 having a lower edge 19 comprising a first longitudinally-extending mounting flange 20 and an upper edge 21 comprising a second longitudinally-extending mounting flange 22. Adhesive such as epoxy is applied along these mounting flanges 22 in order to bond the shear webs 16 to the respective half shells 12a, 12b. 
The relative terms ‘upper’ and ‘lower’ are used herein for convenience and refer to the orientation of the blade as shown in the figures. These terms are not intended to limit the scope of the invention.
Referring now to FIG. 1b, once the shear webs 16 have been bonded to the windward blade shell 12a, adhesive is applied along the second (upper) mounting flanges 22 of the shear webs 16, and along the leading edge 24 and trailing edge 26 of the blade shells 12a, 12b. The leeward mould 10b, including the leeward blade shell 12b, is then lifted, turned and placed on top of the windward blade mould 10a in order to bond the two blade half shells 12a, 12b together along the leading and trailing edges 24, 26 and to bond the shear webs 16 to an inner surface 28 of the leeward blade shell 12b. The step of placing one mould half on top of the other is referred to as closing the mould.
It is important that the shear webs 16 are bonded in the correct position to both the windward and leeward blade shells 12a, 12b. Ensuring the correct position of the shear webs 16 on the windward blade shell 12a is relatively straightforward because the blade mould 10 is in an open configuration when the shear webs 16 are bonded to the windward shell 12a and hence the inner surface 17 of the windward shell 12a is readily accessible. It is more difficult to ensure the correct position of the shear webs 16 in relation to the leeward shell 12b as the mould 10 is in a closed configuration when the shear webs 16 are bonded to the leeward shell 12b and hence the inner surface 28 of the leeward shell 12b is not accessible during the bonding process. Presently, therefore, the position of the shear webs 16 relative to the leeward shell 12b is measured with the mould 10 open and one must rely on the shear webs 16 staying in position on their own during mould closure and squeezing of the adhesive.
Referring now to FIG. 1c, a problem can arise when the mould 10 is closed whereby the shear webs 16 may move slightly relative to the leeward shell 12b. For example, the shear webs 16 may move slightly under their own weight during mould closing or they may be dislodged by contact with the leeward shell 12b. The concave curvature of the leeward shell 12b also has a tendency to force the shear webs 16 together slightly, as shown in FIG. 1c. The extent to which the shear webs 16 may move has been exaggerated in FIG. 1c for illustrative purposes, whereas in practice the extent of movement may be as little as a few degrees or just a few millimeters. Still, small levels of movement of the shear webs 16 during mould closing may result in the shear webs 16 being bonded to the leeward shell 12b at a sub-optimal position.
The present invention addresses this problem.